On a 64 bit system it will use as much as it needs without any practical limit. The typical ram use will depend on your usage, the size of your RAW images, the size of the working sets of images you are working on. If you are quickly flying through images you will see ram usage easily climb into the multigigabyte arena. I generally edit on a portable Mac with 8GB of RAM, but on my Mac Pro I have noticed that going from 8 to 16 GB noticeably increased the speed of Lightroom but one has to wonder whether it's worth it. I can get the RAM usage of Lightroom to go into the +8GB region on that machine. More is better in general but 8GB is probably enough. If you're already at 8GB, you might gain more by going to a SSD for your drive.w

Really? How do you "fly through images"? Memory management on a Mac might be different/broken, but on a fast Windows machine I can lean on the cursor-right key and "fly" through my catalog in develop mode without memory use ever reaching one gigabyte.

Whilst larger amounts of RAM may speed up the process of building preview files, I can't see how it would help flying through images once they are built.

I have a laptop with not much RAM but a SSSD where the previews are stored and this is faster for viewing previews than my desktop with 16 GB of RAM, but no SSSD when it comes to flying through images. As I rarely do much more than quickly view image files on a laptop I think a SSSD is the most beneficial thing you can have. However when it comes to actually working with the files a i7 processor and a decent amount of RAM do speed things up. But the difference is no where near as great as with something like video editing, which is impossible on the laptop.

LR 3.x doesn't require huge amounts of RAM, but does benefit from better processors and faster hard drives. Rendering files is what uses lots of RAM, so for faster building of previews, faster exporting of files then more RAM does help. I can't see that the OS in use makes much difference to this, although OSX has a better reputation for memory management although I can't say I notice any difference these days and I use both OS's.

I wound up getting an i7 2600K and 16GB of RAM (two pairs of 2x4GB.) I found Kingston HyperX blu RAM on sale for only $40 per 8GB pair, before even counting a mail-in rebate which knocks another $15 off, so I just went for that.

You can view what various memory processes and the overall computer are using via TaskManager and the Resource Monitor.

To start TaskManager I usually just right-click on my task-bar and choose TaskManager.

I usually have have or add Memory (Private Working Set) and Commit Size and PF Delta as columns in the Processes tab of TM plus whatever is there by default. The Resource Monitor is started using a button at the bottom of the Performance tab in TM.

If you have TM set to Always-on-top then it might cover over the Resource Monitor panel so you should minimize or at least move TM to the side.

"I have a laptop with not much RAM but a SSSD where the previews are stored and this is faster for viewing previews than my desktop with 16 GB of RAM, but no SSSD when it comes to flying through images."

That corresponds with my experience. I don't think it's the lack of RAM that is slowing down LR - at least if you have 4 GB or more; it's rather that the hard drive(s) get extremely busy resulting in a increasingly long queue of read & write cycles.

Rather than throwing more RAM at LR it might be more helpful to alleviate the read & write cycles by spreading them out over several physical disks. Partitions of one disks won't do it, because it's the time that the head of the hard drive needs to go from one location to another.

In my experience the following practices help to speed up LR;

1) Put your catalog on a different (physical) drive than where LR resides. You can have the catalog on an external drive, provided it's an eSata drive. USB drives are too slow - or more precise the connection speed is too slow. Remember, the catalog can be on a different drive than where your pictures are located.

2) If you have, put the LR cache on a third physical drive.

3) If you are on a PC, defrag your drive often. The seek times are significantly higher on fragmented drives.

I meant the catalog file (the .lrcat file). As I said, the catalog and the photos do not have to be on the same drive.

The Cache is a space on your hard drive that LR uses as temporary storage space for previews. You can see its location and set its size in >Edit >Preferences > File Handling tab. At the bottom you see the location. Set the size to at least to 25 GB.